1. Field of the Invention
The present invention relates to a current sensing device and a manufacturing method of the same. The present invention can in particular relate to a current sensing device that senses a current in such way that a magnetic sensor inserted in a gap of a core detects a magnetic field that is generated in the core in response to the current.
2. Description of Related Art
There is known a current sensing device that detects a current flowing into or from, for example, a battery of a vehicle and the like. This kind of current sensing device is typically configured as follows. The current sensing device includes a core and a magnetic sensor inserted in a gap formed in a core. The magnetic sensor detects a magnetic field that is generated in the core in response to a current flowing in a conductive wire connected to the battery. In the current sensing device, since a measurement target current (i.e., a current to be measured) is proportional to the density of magnetic flux generated in a core gap, the current sensing device is also called a magnetic proportional type current sensing device.
The inventor of the present application has studied a current sensing device, discussion on which will be given below with the findings of the inventor of the present application.
A core of a magnetic proportional type current sensing device may be made of a soft magnetic material. The soft magnetic material can be slightly magnetized by the magnetic field that is generated in response to the measurement target current.
When the core is magnetized, the magnetic filed is generated in the core gap even when the measurement target current does not flow. An output of the magnet sensor can have an offset. In order to reduce the offset, a material with a small coercivity may be required as a material of the core of the current sensing device.
Permalloy is generally known as a magnetic material with a small coercivity.
Permalloy is however nickel alloy, which is approximately 10 to 50 times as expensive as iron and steel.
In view of the above, the use of inexpensive iron or steel as a core material is studied. In iron and steel, a grain-oriented magnetic steel sheet is a material with a relatively small coercivity. More specifically, the grain-oriented magnetic steel sheet (also called a grain-oriented electrical steel sheet and stripe) is a steel material that has a sheet shape and has a magnetization easy axis in a certain direction thereof. The grain-oriented magnetic steel sheet has a remarkably small coercivity in a direction of the magnetization easy axis.
FIG. 23 is a diagram illustrating a property of a grain-oriented magnetic steel sheet. The illustrated property is magnetic flux density as a function of magnetizing force. FIG. 24 is a diagram illustrating coercivity and saturation magnetic flux density of a grain-oriented magnetic steel sheet in a direction of the magnetization easy axis and a direction perpendicular to the magnetization easy axis. In FIG. 23, the solid line shows the property of the grain-oriented magnetic steel sheet in the direction of the magnetization easy axis, and the dashed line shows the property of the grain-oriented magnetic steel sheet in the direction perpendicular to the magnetization easy axis. In FIG. 23, the coercivity is shown by an intersection point between the horizontal axis magnetizing force and the lines (solid line, dashed line).
As shown in FIG. 24, the permalloy with a nickel content rate of 78% has, as the magnetic property thereof, a coercivity of approximately 1.6 A/m and a magnetic flux density of approximately 0.5 T. The permalloy with a nickel content rate of 45% has, as the magnetic property thereof, a coercivity of approximately 7 A/m and a magnetic flux density of approximately 1.5 T. A material for the core of the current sensing device may need to have a magnetic property provided by the permalloy with a nickel content rate of approximately 45%.
As shown in FIGS. 23 and 24, a grain-oriented magnetic steel sheet can have a coercivity of approximately 5 A/m and a magnetic flux density of approximately 2.0 T in the direction of the magnetization easy axis. In the direction perpendicular to the magnetization easy axis, the grain-oriented magnetic steel sheet can have a coercivity of approximately 180 A/m and a magnetic flux density of approximately 1.3 T. As can be seen from the above, the grain-oriented magnetic steel sheet has, in the direction of the magnetization easy axis, the magnetic property required for the core of the current sensing device.
Hence, when the core is constructed with use of a grain-oriented magnetic steel sheet, it is necessary to form a magnetic path in the direction of the magnetization easy axis.
FIGS. 25A and 25B are diagrams illustrating one example structure of a core made with a grain-oriented magnetic steel sheet.
A core 300 made with a grain-oriented magnetic steel sheet can be formed through the following steps. As shown in FIG. 25A, a grain-oriented magnetic steel sheet 301 is wound in the direction of the magnetization easy axis. As shown in FIG. 25B, an end part 302 of the wound grain-oriented magnetic steel plate 301 is fixed by welding or the like, and then, a gap 304 is formed by cutting a part of the wound grain-oriented magnetic steel plate 301 using a cutter 303 or the like.
FIG. 26 is a diagram illustrating one example structure of a core made with permalloy.
A core 400 made with permalloy can be formed through: press-working a plate made of permalloy into a shape that has a gap 401 and a center hole 402 etc.; and stacking multiple press-worked plates into a multilayer structure.
As can be seen from the above, the core 300 made with the grain-oriented magnetic steel sheet may involves complicated manufacturing processes including a step of winding the grain-oriented magnetic steel sheet, a step of fixing the end part of the wound grain-oriented magnetic steel sheet, a step of forming the gap by cutting the part of the wound grain-oriented magnetic steel sheet, and the like. By contrast, the core 400 made with permalloy can be formed through press-working the plate and stacking the press-worked plates into a multilayer structure. Therefore, although the core 300 made with a grain-oriented magnetic steel sheet can be manufactured with low-priced materials, the core 300 made with a grain-oriented magnetic steel sheet may involve complicated working processes and worse assembly performance as compared to the core 400 made with permalloy.
The inventor of the present application considered that it is necessary to improve assembly performance of a core made with a grain-oriented magnetic steel sheet. Meanwhile, the inventor has found that if a grain-oriented magnetic steel sheet, which has a magnetization easy axis, is press-worked and stacked in a manner similar to the manufacturing of the core made with permalloy, a magnetic flux flows in the stacked grain-oriented magnetic steel sheets in the direction perpendicular to the magnetization easy axis, and the stacked grain-oriented magnetic steel sheets may have a large hysteresis and a small maximum measurement current due to the magnetization.
Japanese Patent No. 3790147 and Japanese Patent Unexamined Application Publication No. 2008-224488 have proposed a core made with a grain-oriented magnetic steel sheet. In such cores, a grain-oriented magnetic steel sheet are press-worked to form two kinds of core members whose magnetization easy axes are perpendicular to each other, and the two kinds of core members are alternately stacked into a multilayer structure so that the averaged magnetic permeability is the same all around each core member.
When the core made with a grain-oriented magnetic steel sheet is manufactured in the above-described way, a core made with a grain-oriented magnetic steel sheet can be manufactured through the substantially same manufacturing processes as the core made with permalloy is manufactured.
In the following, explanation is given on properties of the core that is formed through: press-working an grain-oriented magnetic steel sheet to form two kinds of core members whose magnetization easy axes are perpendicular to each other; and alternately stacking the two kinds of core members into a multilayer structure so that the averaged magnetic permeability is the same all around each core member.
FIGS. 27A to 27C are diagrams for explanation on a magnetic path of a core having stacked core members made of a grain-oriented magnetic steel sheet. In FIGS. 27A to 27C, the magnetic path formed in a core 1100 is represented by the arrow, and the magnetic permeability is represented by contrasting density. In FIGS. 27A to 27C, the high contrasting density represents a portion with a small magnetic permeability, and the low contrasting density represents a portion with a large magnetic permeability.
The core 1100 includes a core member 1110 and a core member 1120 each having a rectangular ring shape. The core member 1110 is processed so to have a magnetization easy axis in a direction of a long side portion thereof. The direction of a short side portion of the core member 1110 is perpendicular to the magnetization easy axis of the core member 1110.
The core member 1120 of the core 1100 is processed so as to have an magnetization easy axis in a direction of a short side portion thereof. A direction of a long side portion of the core member 1120 is perpendicular to the magnetization easy axis.
In the core 1100, the core member 1110 and the core member 1120 are stacked.
FIG. 27C is a diagram illustrating the stacked core members that are, for explanation purpose, rolled out from a gap G in an anticlockwise direction.
Explanation will be given on distribution of magnetic flux in the core members 1110 and 1120, where the magnetic flux is generated by a magnetic field resulting from a current flowing through a center part of the core members 1110 and 1120.
When a comparison is made between the magnetic permeability of the core member 1110 and that of the core member 1120 at the point “a” shown in FIGS. 27A to 27C, the magnetic permeability of the core member 1120 is predominantly larger than that of the core member 1110, and thus, all of the magnetic flux passes through the core member 1120. In an interval from the point “a” to the point “c”, the magnetic permeability of the core member 1120 is on the decrease while the magnetic permeability of the core member 1110 is on the increase.
At the point “b”, the magnetic permeability of the core member 1110 and that of the core member 1120 become approximately equal to each other. Thus, in the vicinity of the point “b”, the magnetic flux is approximately uniformly distributed in the core members 1110 and 1120. As closer to the point “c”, the magnetic permeability of the core member 1110 is larger. Thus, in an interval between the point “c” and the point “d”, the magnetic flux is moved to the core member 1110.
After the magnetic flux passes through the interval between the point “c” and the point “d” of the core member 1110, the magnetic flux is moved to the core member 1120 in an interval between the point “d” and the point “e” because of a decrease in magnetic permeability of the core member 1110 and an increase in magnetic permeability of the core member 1120. In an interval between the point “f” and the point “g”, all magnetic flux is moved into the core member 1120. In a manner similar to the above, the magnetic flux is moved in an interval between the point “g” and the point “i” and an interval between the point “k” and the point “n”.
As can be seen from the above, when the core members 1110, 1120 each made of a grain-oriented magnetic steel sheet are stacked so that the magnetization easy axis of the core member 1110 and that of the core member 1120 are perpendicular each other, the magnetic flux generally flows in a portion of the core member 1110 or 1120 in the direction of the magnetization easy axis, and the magnetic flux generally does not flow in a portion of the core member 1110 or 1120 in the direction perpendicular to the magnetization easy axis.
Although it is ideal that the core member 1110 and the core member 1120 are assembled so as to firmly stick each other, a tiny clearance exists in a practical sense. Since the magnetic permeability of the clearance is approximately equal to that of vacuum, the clearance acts as a resistance for the magnetic flux to move between the core member 1110 and the core member 1120. Since the magnetic resistance is larger with increasing clearance, a part of the magnetic flux may not move between the core member 1110 and the core member 1120 in the case of a large clearance.
This has a negative influence on magnetic saturation properties of a core generally required in a current sensing device as a whole. For example, a maximum measurement current is lowered. Coercivity is worsened, that is, hysteresis is increased.